This invention relates to a stacked-type electronic device such as a stacked-type ceramic capacitor, a stacked-type piezoelectric ceramic actuator, and others having a ceramic body with a plurality of internal electrodes and an external electrode formed on the ceramic body and connected, as a lead terminal, to the internal electrodes, and, in particular, to such a stacked-type ceramic device having a fuse element for breaking abnormal current.
A typical one of the stacked type electronic devices is a ceramic capacitor which comprises a ceramic body of a ceramic material with first and second sets of internal electrodes and first and second external electrodes formed on the ceramic body at different positions. The first set of internal electrodes and the second set of internal electrodes are alternately disposed in the ceramic body and spaced from each other by the ceramic material. The first and second sets of internal electrodes are connected to the first and second external electrodes are connected to the first and second internal electrodes. The first and second external electrodes serve as lead terminals for connecting the capacitor to an external circuit.
The first set of internal electrodes and the second set of internal electrodes will often be referred to as first internal electrodes and second internal electrodes, respectively, hereinafter.
In production of the ceramic capacitor, first and second internal electrodes are formed by screen printing method on ceramic green sheets respectively. Then, green sheets having first internal electrodes and green sheets having second internal electrodes are alternately stacked to form a ceramic body. The ceramic body is fired or baked to form a hard ceramic body having the first and second internal electrodes. Then, external electrodes are formed on the ceramic body.
In the stacked-type capacitor, when adjacent ones of the first and second internal electrodes are short-circuited by any fault, for example, pin holes existing in the ceramic body, an abnormal current flows between the first and second external electrodes through the short-circuited internal electrodes to cause destruction of the external circuit as well as the capacitor itself. This is a serious problem.
The stacked-type piezoelectric ceramic actuator has a similar structure of the capacitor as described above. The ceramic material is a piezoelectric material and is polarized between the adjacent first and second internal electrodes. The stacked-type piezoelectric ceramic actuator has also the similar problem.
Generally speaking, the electronic devices having a ceramic body with a plurality of internal electrodes connected to an external electrode also suffers from the similar problem.
In order to resolve the problem, JP-A 63-305506 (Reference I) discloses to form a narrow width portion in each of the internal electrodes adjacent the external electrode. The narrow width portion is referred to as a fuse electrode. The fuse electrode is melted and broken out by the Joule heat caused by the abnormal electrode flowing therethrough.
However, there is another problem that the Jule heat may cause the ceramic body itself cracked.
JP-A 3-1514 (Reference II) discloses to use a wire-shape fuse element for connecting the internal electrodes with the external electrode. An outer electrode is formed on an outer surface of the ceramic body and is connected to the internal electrodes. The outer electrode is connected to the external electrode by the wire-shape fuse element.
It is troublesome that the wire-shape fuse element extends on the outer surface of the ceramic body. Further, when a pair of adjacent internal electrodes is only short-circuited, the fuse element is broken out. This means the capacitor can no longer be used.
Therefore, it is an object of this invention to provide a stacked-type ceramic electronic device having a plurality of internal electrodes connected to an external electrode and a fuse function for protecting against abnormal current, wherein that ones of the internal electrodes through which the abnormal current flows are only removed or separated by the fuse function from the connection between the internal electrodes and the external electrode without any crack being caused in the ceramic body, so that the device can then be operable while still maintaining its electronic properties.
It is another object of this invention to provide the stacked-type ceramic electronic device with the fuse function which is of a surface mount type.
It is another object of this invention to provide the stacked-type electronic device with the fuse function which has an improved equivalent series resistance.
According to the present invention, a stacked-type electronic device is obtained which comprises:
a ceramic body of a ceramic material having an outer surface including a side surface;
a set of internal electrodes disposed in the ceramic body and spaced from each other by the ceramic material, the internal electrodes extending in parallel with each other, the internal electrodes having their individual lead-out electrodes extending therefrom to their end surfaces exposed in the side surface;
an external electrode mounted on the ceramic body for connecting to an external electric circuit;
a film electrode connected to the external electrode, the film electrode being deposited on the side surface and overlying the end surfaces of the individual lead-out electrodes in common to be directly connected to the lead-out electrodes.
At least, the film element partially fuses by a heat caused due to an abnormal current flowing therethrough from the external electrode to a particular one of the internal electrodes to break the abnormal current from flowing to the particular internal electrode.
In an aspect, the film electrode comprises metallic material having a melting point lower than that of the internal electrodes.
In an embodiment, the film electrode has a thickness of 0.5-50 micrometers (xcexcm). Each of the lead-out electrodes has a width of 50-800 micrometers (xcexcm).
In an example, it can make use of metals or alloys selected from a group of Ag, Pd, Cu, Ni, Sn, Zn, Bi, Pb and Cd for the metallic material of the film element.
In a preferred embodiment, the film electrode is a baked film formed by an electrode metal paste being coated on the side surface and then baked.
Preferably, the electrode metal paste is a metallic powder paste comprising electrode metallic powder and a carrier solvent suspending the powder therein.
More preferably, the metallic powder paste further contains at least one selected from glass forming elements of SiO2, Al2O3 and B2O3.
Alternatively, the electrode metal paste is an metallo-organic compound past which comprises resinate of metallo-organic compound/organometallic compound and organic solvent resolving the resinate therein. The reginate of the metallo-organic compound/organometallic compound is one comprises the metallo-organic compound/organometallic compound and resin mixed to the the metallo-organic compound/organometallic compound.
The metallo-organic compound preferably contains at least one of glass forming elements of Si, Al, and B.
The electrode metal paste may be a mixed paste which comprises the metallic powder paste and the metallo-organic compound paste mixed with the metallic powder paste. It is preferable that the metallic powder paste further contains at least one selected from glass forming elements of SiO2, Al2O3 and B2O3, or the metallo-organic compound may contain at least one of glass forming elements of Si, Al, and B.
In an embodiment of the stacked-type electronic device according to this invention, the external electrode overlaps the film electrode. The external electrode and the film electrode can be made of the same material.
Alternatively, the external electrode and the film electrode can be spaced from each other by a hollow space formed therebetween, or an insulating glass layer may partially be interposed between the external electrode and the film electrode.
In another embodiment, the external electrode is formed at a position on the outer surface generally different from the film electrode.
In the embodiment, the external electrode and the film electrode are located at spaced positions on the outer surface, and a connecting electrode is deposited on the outer surface between the spaced positions and connecting the external electrode with the film electrode.
Alternatively, the external electrode and the film electrode are partially but slightly overlapped to each other. The film electrode is covered with a protective glass layer. The protective glass layer has a melting point lower than that of the film electrode but is higher than a soldering temperature for soldering the external electrode to the external circuit.
In the stacked-type electronic device of this invention as described above, the side surfaces, the set of internal electrodes, the lead-out electrodes, the external electrode and the film electrode are a first side surface, a first set of internal electrode, first lead-out electrodes, a first external electrode, and a first film electrode, respectively. The device may further comprise a second set of internal electrodes disposed in the ceramic body having their individual second lead-out electrodes with their ends exposed in the outer surface at positions spaced from the first lead-out electrodes and the first external electrode, and a second external electrode formed on the ceramic body and spaced from the first lead-out electrodes and the first external electrode. But the second external electrodes are electrically connected to the second set of internal electrodes in common.
The stacked-type electronic device may further comprises a second film electrode connected to the second external electrode. The second film electrode is deposited on the outer surface and overlying the ends of the individual second lead-out electrodes in common to be directly connected to the second lead-out electrodes. At lest the second film electrode may partially fuse by a heat caused due to an abnormal current flowing therethrough from the second external electrode to a particular one of the second set of internal electrodes to break the abnormal current from flowing to the particular one of the second set of internal electrode.
The outer surface may comprise the first side surface and two opposite end surfaces at opposite ends of the first side surface, and the first and second external electrodes are formed on the first and second end surfaces.
The second lead-out electrodes are exposed in the first side surface. A distance between the second lead-out electrodes and the second external electrode is same as another distance between the first lead-out electrodes and the first external electrode.
Alternatively, the outer surface further comprises a second side surface disposed opposite to the first side surface and between the first and second end surfaces. The second lead-out electrodes are exposed in the second side surface.
Further, a first and a second dummy film electrodes may be disposed on the second and the first side surfaces opposite to each other and at positions opposite to the first and the second film electrodes, respectively.
An example of the stacked-type electronic device according to this invention is a stacked-type capacitor wherein each of the first set of internal electrodes and each of the second set of internal electrodes are alternatively disposed in the ceramic body.
Another example of the stacked-type electronic device according to this invention is a stacked-type piezoelectric ceramic actuator wherein each of the first set of internal electrodes and each of the second set of internal electrodes are alternatively disposed in the ceramic body, and the ceramic body is polarized in a stacked direction between adjacent electrodes.